After a well has been completed to produce oil or gas, it is necessary to periodically service the well. There are many occasions when the service procedure is carried out using coiled tubing. Such tubing is inserted into the wellhead through a lubricator assembly or stuffing box. Typically, this is necessary because there is a pressure differential at the surface of the well and the atmosphere, which may have been naturally or artificially created, that serves to produce oil or gas or a mixture thereof from the pressurized well. The tubing that is inserted into the well is normally inserted through a lubricator mechanism which provides a seal about the O.D. of the tubing for the retention of any pressure that may be present at or near the surface of the well. The tubing is inserted by a coiled tubing injector apparatus which generally incorporates a multitude of gripper blocks for handling the tubing as it passes through the injector. The tubing is flexible and can therefore be cyclically coiled onto and off of a spool, or reel, by the injector which often acts in concert with a windlass and a power supply which drives the spool, or reel.
The injector utilizes a pair of opposed endless drive chains which are arranged in a common plane. These opposed endless drive chains are often referred to as gripper chains because each chain has a multitude of gripper blocks attached therealong. The gripper chains are driven by respective drive sprockets which are in turn powered by a reversible hydraulic motor. Each gripper chain is also provided with a respective idler sprocket to maintain each gripper chain within the common plane. Both the drive sprockets and idler sprockets are mounted on a common frame wherein the distance between centers of all the sprockets are essentially of a constant distance from each other. That is, the drive sprockets are free to rotate, but are not free to move either vertically or laterally with respect to each other. The idler sprockets are not free to move laterally with respect to each other, but are vertically adjustable within a limited range in order to set the amount of play in each gripper chain. Such vertical adjustment is made by either a mechanical adjusting means or a hydraulic adjusting means. Typically, for injectors having mechanical adjustment means, the adjustment is made when the injector is not in operation.
The opposed gripper chains, preferably via the gripper blocks, sequentially grasp the tubing that is positioned between the opposed gripper chains. When the gripper chains are in motion, each gripper chain has a gripper block that is coming into contact with the tubing as another gripper block on the same gripper chain is breaking contact with the tubing. This continues in an endless fashion as the gripper chains are driven to force the tubing into or out of the wellbore, depending on the direction in which the drive sprockets are rotated. Gripper blocks such as those set forth in U.S. Pat. No. 5,094,340, issued Mar. 10, 1992, to Avakov, which is incorporated herein by reference, may be used.
The gripper chain is provided with a predetermined amount of slack which allows the gripper chain to be biased against the tubing to inject the tubing into and out of the wellbore. This biasing is accomplished with an endless roller chain disposed inside each gripper chain. Each roller chain engages sprockets rotatably mounted on a respective linear bearing beam, referred to herein as a linear beam. A linkage and hydraulic cylinder mechanism allows the linear beams to be moved toward one another so that each roller chain is moved against its corresponding gripper chain such that the tubing facing portion of the gripper chain is moved toward the tubing so that the gripper blocks can engage the tubing and move it through the apparatus. The gripper blocks will engage the tubing along a working length of the linear beam.
Each gripper chain has a gripper block that contacts the tubing at the top of the working length as a gripper block on the same chain is breaking contact at a bottom of the working length of the linear beam.
The fixed distance between each set of drive sprockets and idler sprockets requires some significant lateral movement in the gripper chain when engaged by the roller chain on the corresponding linear beam in order to allow the gripper chains to engage the tubing by way of the gripper blocks. The reason for having the requisite amount of lateral play in the gripper chains is to provide a limited amount of clearance between the gripper chains, upon moving the respective roller chains away from the vertical centerline of the injector, to allow the passage of tubing and tools having larger outside diameters or dimensions. One shortcoming in this design is that the required slack can often cause misalignment problems and even binding problems with the gripper chains due to having to accommodate ever increasing outside nominal dimensions of downhole tools and wellhead equipment.
There are, within the art, injectors that can accommodate a variety of outside diameters of tubing while avoiding: chain misalignment, chain binding tendencies, improper chain tension, gripper block marking or gouging, and other inherent design problems of prior injectors which manifest themselves when working with tubing, tools, and surface equipment having large nominal outside diameters. Examples of such injectors are disclosed in U.S. Pat. No. 5,553,668, issued Sep. 10, 1996, to Council et al., and U.S. Pat. No. 6,209,634, issued Apr. 3, 2001, to Avakov et al., both of which are incorporated herein by reference and assigned to the assignee of the present invention.
There are, however, other difficulties that are faced when utilizing an injector to inject, suspend, or extract tubing from a wellbore. During operation of an injector, the idler sprockets may move, so that the centerline of the idler sprocket moves relative to the centerline of the drive sprocket, and may no longer be parallel to the centerline of the drive sprocket. There is a need for an apparatus that will allow the distance between the drive sprockets and the idler sprockets to be monitored and verified.
Generally, as provided herein, the wellbore in which the tubing is injected will be pressurized, so that as the tubing is initially inserted through the injector and into the wellbore, the pressure will tend to resist injection of the tubing. The pressure in the well attempts to push the tubing upwardly as the tubing is being injected, which causes buckling forces to be applied to the tubing between the lower end of the working length of the gripper chain and the point at which the tubing is engaged in the stuffing box or lubricator. Further, the tubing should be lubricated prior to entering the stuffing box. Presently, a wetting fluid is manually applied to the tubing after the tubing is spooled off the reel and prior to the time the tubing is engaged by the injector. Manual application of wetting fluid can be hazardous to the environment, due to the nature of the wetting fluids that may be used, and may be dangerous to the workers applying the fluid.
It is also important that as the downward forces are applied to the tubing by the gripper chains and the gripping loads are applied to the tubing, the gripper chains remain parallel to one another, and resist transverse movement. If the gripper chains move sideways relative to one another, the result can be less gripping force, and damage to the tubing and the injector.